Maximum/minimum thermometer

ABSTRACT

This invention relates to maximum/minimum thermometer of the type comprising an expansion liquid, a transfer liquid and indexes for recording the maximum and minimum temperatures. Described in particular are new transfer liquids designed to replace conventionally used mercury and new indexes suitable for use in the disclosed thermometers.

This application claims priority from Great Britain Application No.9905216.9 filed Mar. 9, 1999 and PCT Application No. PCT/GB00/00833,filed Mar. 7, 2000, both in English, the disclosures of which areincorporated by reference herein in their entirety.

This invention relates to maximum/minimum thermometers.

Maximum/minimum thermometers are well known in the art and date backsome two hundred years. For example that of J. Six (1731–1793) wasdescribed by him to the Royal Society in 1782.

U.S. Pat. No. 4,627,741 discloses a typical maximum/minimum thermometerwhich has an improved index recovery mechanism.

As will be appreciated maximum/minimum thermometers such as described inU.S. Pat. No. 4,627,741 are often constructed from a U-shaped liquidcontaining tube in which mercury metal is provided in the bend of theU-section and extends into the two legs of the U-tube. The mercuryserves as a so-called “transfer liquid”.

In addition to the mercury, the U-tube contains an expansion liquidwhich contacts the mercury, and is immiscible therewith. Movement of themercury is effected by the expansion or contraction of the expansionliquid in response to changes in temperature.

Also provided are two temperature display indexes or pins containedwithin the U-shaped tube. In use the indexes are initially positioned sothey contact and float at the surface of the mercury. Upon advancementof a mercury front, the index in contact with the advancing mercuryfront is moved along within the U-tube.

The indexes may be described as unidirectional devices since uponretreat of the mercury front from the index, the index retains itsposition in the U-tube typically by virtue of a restraining method. Forexample one method of restraint is provided by a magnetic attractiveforce between the index and a magnetic strip placed adjacent to theU-tube.

An alternative index restraining method may be provided by a frictionalforce between the inner wall of the U-tube and a sprung glass hairattached to the index.

By using two indexes, one at each front of the mercury, the recordal ofthe temperature at each extreme end of a temperature range is thuspossible. One index will accord a position which represents the maximumtemperature, and the other index will accord a position representing theminimum temperature observed over a particular time interval.Temperature markings are provided on a graduated scale positionedadjacent to the U-tube allowing the maximum and minimum temperatures tobe read directly.

Currently however, the transfer liquid of choice is mercury metalbecause of its suitable physical properties such as its high density,(i.e. 13.6 gcm⁻³) and its immiscibility with many expansion liquids.Moreover mercury metal also remains liquid, over a wide temperaturerange (ie. mercury has a melting point of −38.9° C. and a boiling pointof 356.6° C.), making it suitable for use in many maximum/minimumthermometer applications.

Notwithstanding the aforementioned suitable characteristics of mercuryit is apparent, because of current heightened environmental awareness,that use of mercury in thermometers is undesirable. For instance mercuryis highly toxic. Thus, mercury requires special handling and disposalprocedures should spillage occur, for example through thermometerbreakage. Moreover, mercury is also relatively expensive.

It is among the objects of the present invention to obviate or at leastmitigate one or more of the aforementioned problems associated withthermometers which employ mercury as a transfer liquid.

It is an object of the present invention to provide a maximum/minimumthermometer comprising a transfer liquid which is substantially or morepreferably entirely mercury free.

According to a first aspect of the present invention there is provided amaximum/minimum thermometer comprising an expansion liquid which expandsor contracts in response to changes in temperature, a transfer liquidwhich is substantially immiscible with the expansion liquid, and twoindexes capable of being moved through action of the transfer orexpansion liquid, wherein the transfer liquid is mercury free.

It is understood that the term “transfer liquid” refers to the liquidwithin a maximum/minimum thermometer which is moved in response to thethermal expansion/contraction of the expansion liquid, and the purposeof which is to assist in identifying the maximum and minimumtemperatures over a particular time period, by providing a means formoving the indexes to positions indicative of the maximum and minimumtemperatures.

The maximum/minimum thermometer may be of a typical U-tube designwherein the transfer liquid is positioned within the bend of the U-tube,extending into each leg thereof. In use, the indexes initially rest ateach front of the transfer liquid. The maximum and minimum temperaturesmay be read off a temperature scale associated with the U-tube from thefinal resting positions of the indexes.

It is generally understood that the transfer liquid should have thefollowing properties:

-   i) be immiscible with the expansion liquid;-   ii) remain substantially in a mobile liquid state at and between the    upper and lower temperature limits of the thermometer in use; and-   iii) have a density which is different from that of the expansion    liquid.

To ensure that the transfer liquid remains in the liquid state withinany particular temperature range, the skilled addressee will appreciatethat the contents of the U-tube may be pressurised accordingly. Forexample for any particular transfer liquid an increase in pressure willenable the liquid state to exist at higher temperatures.

Desirably the transfer liquid may also have:

-   low thermal expansivity;-   display low toxicity; and-   be coloured or be capable of being coloured.

Depending on any particular application the skilled addressee may chooseany suitable combination or all of the above-mentioned desirableproperties in addition to the generally essential properties i), ii) andiii).

Typically the transfer liquid will have a density greater than theexpansion liquid. Thus, for example, if a U-shaped maximum/minimumthermometer is used in an upright position, that is, with the legs ofthe U-tube directed upwards, the transfer liquid will rest below theexpansion liquid traversing the bend in the U-tube and extending intothe two legs. However, it should be understood that maximum/minimumthermometers may be designed which rest sideways, rather than in theupright position, or maybe of other shapes.

It will be appreciated that for a thermometer which is to be used in anupright position the choice of transfer liquid may be determined by thedensity of the expansion liquid being used. For example if toluene(density 0.87 g.cm⁻³), a typical expansion liquid, is used, the transferliquid will have a density greater than 0.87 g.cm⁻³.

Maximum/minimum thermometers are often used to measure a maximum/minimumair temperature for example in a greenhouse. Thus, the transfer liquidshould preferably remain substantially liquid up to and above thehighest expected temperature. Therefore, the transfer liquid willgenerally be required to remain substantially liquid up to and above 50°C., more preferably above 70° C. It will be appreciated however that thechoice of transfer liquid will ultimately depend on the application themaximum/minimum thermometer is being put to. Consequently the transferliquid may be chosen for thermometer use at much higher temperatures forinstance above 150° C.

Conversely the transfer liquid should preferably remain substantiallyliquid at a temperature which is below that of the lowest temperatureexpected to be encountered when using a maximum/minimum thermometer.Thus, for example, for outside use the substantially liquid state of thetransfer liquid should be maintained preferably below −30° C., morepreferably below −50° C.

The skilled addressee will appreciate that environmental ambienttemperature changes generally occur slowly, and therefore this allowsfor a viscous transfer liquid to be used if necessary. A viscous liquid,although being mobile, will move relatively slowly. Alternatively, thetransfer liquid may not be completely homogeneous, that is, it maycontain suspended solid particles or droplets of liquid; such a transferliquid may be colloidal.

Preferably the transfer liquid displays low thermal expansion. Thus,although an expansion liquid expands/contracts in response totemperature changes, it is preferable that the transfer liquid displaysa low degree of expansion/contraction in response to temperaturechanges. However, if the transfer liquid does display a certain degreeof thermal expansion this may be taken account of by appropriate designof the temperature scale. Such a design may be a non-linear temperaturescale.

Since the transfer liquid is moved within the U-tube byexpansion/contraction of the expansion liquid, the transfer liquid mustbe substantially immiscible with the expansion liquid and/or displaysubstantially no chemical interaction with the expansion liquid.

It is also desirable that the transfer liquid display low toxicity, orat least be considered less toxic than mercury.

Although it is the indexes which generally serve to identify the maximumand minimum temperatures, it is preferable aesthetically that thetransfer liquid is coloured or is capable of being coloured so that itmay easily be seen in the thermometer. Typically the transfer liquid maybe coloured by the addition of suitable dyes. Depending on the transferliquid being used suitable dyes may include Eurocert Green S orwater-soluble salts of inorganic complex ions such as the tetra-amminocopper (II) ion or the hexacyanatoferrate ion. Naturally, the skilledaddressee may envisage the use of other suitable dyes.

A suitable dye may be chosen for its preferential solubility of the dyein the transfer liquid as compared to its non-solubility in theexpansion liquid.

Examples of suitable transfer liquids which possess at least some of theaforementioned preferred properties include solutions comprisinginorganic or organometallic compounds, for example solutions ofcompounds of elements found in groups III, IV, V, VI and VII of theperiodic table (see table 1 below) or mixtures thereof, as well asorganic liquids, such as members of the halogenoparaffin series ofcompounds.

TABLE 1 Group Possible Compounds III Halides of boron IV Halogenderivatives of silane and disilane, Mixed silicon halides andorgano-halides, Many possible compounds derived from stannanesubstituted with halogens and alkyl groups. Organolead compounds.Titanium tetrachloride V Pure and mixed halides and organohalides ofphosphorus. Halogen substituted derivatives or arsine and stibine VISulphur halides, some selenium and tellurium halides VII Someinterhalogen compounds

Additionally, a recent revival of interest in molten salt chemistry hasled to the exploration and development of low-melting ionic liquids.These are salt mixtures which are liquid at room temperature andexamples exist which are liquid down to −90° C. A number of possiblesystems already exist and many more can be envisaged. Most currentknowledge relates to binary systems containing the compounds aluminiumtrichloride, phosphorous pentafluoride, and boron trifluoride. Whenmixed with quaternary ammonium salts in which the substituents may beparaffin chains of various lengths, a range of liquid results. In suchsalts the quaternisable nitrogen atoms may be in rings such as pyridineor imidazole rings.

As examples it is possible to produce a series of such low-meltingliquids by combining 1-alkyl-3-methyl imidazolium chloride with borontrifluoride via metathesis of the chloride of the former with sodiumtetrafluoroborate.

It is envisaged that liquids having densities in the useful rangementioned in other sections of this patent can by synthesised. The meltsare of low toxicity and may be coloured with standard organic dyes.Certain of the liquids may be employed in the present invention astransfer liquids along with certain of the expansion liquids chosen fromthose mentioned herein.

A particularly preferred transfer liquid is a solution comprising ioniccompounds, typically a solution comprising at least one alkaline earthor alkali metal salt.

The solution may be an aqueous solution in which said at least onealkaline earth and/or alkali metal salt is dissolved. The solution mayhowever comprise an organic liquid, such as an alcohol, ketone, ether ormixtures thereof.

Alkaline earth and/or alkaline metal salts are particularly preferredbecause of their high solubilities in water. For example it is possibleto make aqueous solutions in which the amount of alkaline earth and/oralkali metal salt is between 80% and 400% w/v, for example between 100%to 200% w/v.

Dissolution of the alkaline earth and/or alkali metal salt in waterlowers the freezing point below 0° C. The degree of freezing pointlowering will depend on the molecular weight of the solute, itspercentage concentration in solution, and the number of ions into whichit dissociates in aqueous solution. Each “particle” in the solutioncontributes separately to the freezing point lowering (the Van't Hoffi-factor). This makes alkaline earth and/or alkali metal salts veryefficient in lowering the freezing point. Using solids of such highsolubility it is possible to lower the freezing point of a concentratedaqueous solution to −40° C. to −50° C.

Such salt solutions comprising high percentages of alkaline earth and/oralkali metal salts also have densities greater than water. For examplecalcium bromide solutions of 110% w/v, 120% w/v and 125% w/v havedensities of 1.62, 1.66 and 1.68 g.cm⁻³ respectively. The temperature atwhich solid precipitates from each of the solutions, either by freezingor deposit of solute is around −51° C.

Salt solutions such as those described herein also boil above 100° C.since the dissolved substance also raises the boiling point of water inaccordance with Raoult's Law. This makes the substances eminentlysuitable for use as the transfer liquid since the boiling point issubstantially above that of the highest temperature on a typicalmaximum/minimum thermometer scale.

Moreover these aqueous salt solutions are likely to have only limitedself-expansion since the salt is present as ions. The expansivity will,therefore, be similar to that of water itself which, in turn, is only alittle more than mercury and about a fifth that of toluene, a commonthermometer expansion liquid.

If the transfer liquid is water-based there will be an interfaceinteraction only with other liquids with which water is miscible orreacts directly. Such liquids are generally inorganic or are oxygenatedorganic compounds, e.g. alcohols, ketones, phenols, or, possibly,organohalogen compounds. This means that there is a range of otherorganic liquids which may be used as the expansion liquid, for examplehydrocarbons such as toluene, xylenes, kerosene, gasoline, or theirmixtures.

Additionally, dissolved alkaline earth or alkali metal salts havevarying levels of toxicity but a number are essentially non-toxic and,because of the high boiling points of the solutions, have low vapourpressure. Accidental breaking of the thermometer would not, therefore,constitute a hazard and any residue could readily be flushed away withwater.

Generally the alkaline earth or alkali metal salt may be selected fromsuch compounds as halides (for example fluorides, chlorides, bromides,iodides), sulphates, hydroxides, carbonates, chlorates, dichromates,chromates, carboxylates (for example acetates), nitrates, nitrites,phosphates, ammonium compounds or other salts comprising an alkalineearth or alkali metal ion and a cationic species.

Suitable examples of metal salts which may be used are CaI₂, CaBr₂,CsBr, CsF, CsOH, Cs₂SO₄, CH₃COOCs, KF.2HO₂ HCOOK, KI, KNO₂, RbF,NaClO₄.H₂O, Na₂Cr₂O₇.2H₂O or NaI.2H₂O.

Particularly preferred salts are selected from alkaline earth metalhalides, such as CaI₂ and CaBr₂.

Generally speaking solutions of the above-mentioned salts are colourlessor only weakly self-coloured. However, colourants may be readilyintroduced by the use of water-soluble, non-toxic dyestuffs duringpreparation.

It will be appreciated that the same properties may also be achievedusing mixtures of the aforementioned salts in which there is no mutualreaction.

It may also be possible to employ a high-density organic compound astransfer liquid. These are almost exclusively halogenated compounds andcertain of them are toxic. If such an organic material were used,however, the nature of the expansion liquid would have to be changed tosuit the interface (ie. be immiscible). Furthermore, since organicliquids generally have a high thermal expansivity, a significantcorrection to one side of the thermometer scale would be necessary.Mixtures of organic liquids would also be feasible and would be governedby the same principles.

Preferably however the transfer liquid will have a low thermalexpansivity compared to the expansion liquid. Nevertheless to accountfor any small changes in volume in the transfer liquid the temperaturescale of the thermometer may be graduated in a non-linear fashion so asto correct any errors which would otherwise occur if the temperaturescale was linear.

The present invention also in a further aspect provides the use ofsolutions as described herein as transfer liquids in maximum/minimumthermometers.

Desirably the expansion liquid should possess:

-   i) high thermal expansivity;-   ii) be substantially immiscible with the transfer liquid;-   iii) remain substantially in a mobile liquid state at and between    the upper and lower temperature limits of the thermometer; and-   iv) have a density which is different to that of the transfer    liquid.

Given the above specifications, a hydrocarbon or a mixture ofhydrocarbons may be used as the expansion liquid should the transferliquid be an aqueous solution. Hydrocarbons are sharply separable fromwater and can be mixed readily with organohalogen compounds in order toraise the density smoothly.

Hydrocarbons, such as toluene, the xylenes, kerosene, gasoline, or theirmixtures and a variety of other organic compounds could be used eitheralone or in admixture depending on the system chosen. These are cheap,readily available, and often of low toxicity. If the transfer liquidwere itself organic then a different choice of expansion liquid would benecessary. An alternative choice for this role in the latter case couldbe a hydroxylated/oxygenated organic compound such as an alcohol, ether,ketone, or ester or a mixture of these.

The indexes must be designed to function appropriately with the chosentransfer liquid and expansion liquid. Preferably the indexes should bechemically and physically stable towards both the transfer and expansionliquids.

Generally speaking the indexes will have a density less than that of thetransfer liquid and be able to float at the surface of the transferliquid. Such indexes are therefore moved by action of the transferliquid. The indexes may be magnetic or rendered magnetic such that it ispossible for a magnet or magnets placed adjacent or near to thermometerto hold the indexes in position. Retention of an index by the magnetoccurs after retreat of the transfer liquid from said index.

For example it is possible to construct a glass-based index for thethermometer by forming a glass tube with very thin walls and inserting asmall piece of ferrous wire. The overall density of the index can bepre-determined by judiciously combining the density contributions ofglass, wire, and the air contained in the tube.

A suitable index may be fabricated by using a low-density tubularplastic material (e.g. low or high-density polyethylene, polypropyleneor other material having density in the region of 1.0 gm.cm⁻³)containing a piece of ferrous metal. As for the glass-based index abovethe dimensional parameters i.e length, diameter, and wall thickness canbe varied to produce an overall index having suitable density. Theexpansion liquid density can then be tuned to that of the index forbuoyancy; an appropriate index may have density in the region of 1.0gm.cm⁻³.

A third type of solid index can be constructed by first synthesising aplastic with intrinsic magnetic properties. This may be done by mixingand/or melting together the chosen plastic or its components with aquantity of a magnetic powder such as iron filings, magnetite (Fe₃O₄),strontium ferrite, or other magnetic powders. The amounts of eachcomponent may be adjusted to provide suitable magnetic strength whilstpreserving a low density e.g 10–15% of magnetite with polypropyleneproduces a plastic of density 0.98–1.02 gm.cm⁻³. This plastic isinitially produced as a solid mass and then may be melted, extruded, orpulled out to give a fine fibre. This is then chopped to afford magneticplastic indices. Density adjustment of the expansion liquid to suit theindex density must then be carried out.

The magnets for use with a maximum/minimum thermometer comprisingmagnetic indexes generally lie behind the tube legs and the transferliquid must be able to push the indexes along whilst they are in themagnetic field yet leave them “fixed” in position when the transferliquid retreats. The former is generally achieved via buoyancyadjustment of the expansion liquid in a way which makes it possible tomove the index along by only a small “push” of the transfer liquid. Forthe same reason, only a small magnetic attraction is required to holdthe index in place when the transfer liquid retreats.

It should be appreciated however that other indexes such as the typeusing a spring glass hair mentioned in the introduction or otherfrictional devices may also be used. Moreover, if the U-tube is used ina horizontal position, the indexes may not require frictional ormagnetic features. The indexes have to be able to be pushed along by thetransfer liquid and may be reset by tilting the thermometer upright andallowing the indexes to return into contact with the surface of thetransfer liquid.

As mentioned above, the density of the index should generally be lessthan that of the transfer liquid so that the index will rest at thesurface of the transfer liquid. However, fluid statics dictates thatthere will be a partial sinking of the index in the higher densitysurface to a depth determined by the relative densities of the two. Thisis generally aesthetically undesirable in a maximum/minimum thermometerbut may be avoided by raising the density of the expansion liquidappropriately. This may be done by mixing into the principal expansionliquid an amount of a higher-density liquid. For example if toluene(density 0.87 gm.cm⁻³) is used as the main component of the expansionliquid, bromobenzene (density 1.491 gm.cm⁻³) or 1,3-dibromopropane(density 1.989 gm.cm⁻³) may be added. This increases the buoyancy of theexpansion liquid and allows the index to rest on the surface of thetransfer liquid.

The maximum/minimum thermometers described herein are generally formedfrom glass. However, suitable plastics material may also be used.

It has been observed that the transfer liquid may not move completelyfreely within the thermometer and may in some instances tend to stick tothe glass. As such it may be desirable to add a friction reducing agent,such as a surfactant, to the transfer liquid.

The present invention will be further understood with reference to thefollowing detailed description of “test apparatus” showing theunderlying principles of the present invention and with reference toFIGS. 1 and 2, which shows such “test apparatus”.

A maximum/minimum thermometer test apparatus (1) shown in FIG. 1 hasbeen constructed to demonstrate the principles given and to allowexperimentation with both transfer and expansion liquids and withvarious index types. The apparatus (1) is of a U-shaped tube (3) design.The bore of the U-tube is 5 mm. and has at its two ends a reservoir bulb(5) (ca. 6 ml. capacity) and a stopper (7) which are removable forcleaning and re-filling.

In this example the U-tube (3) is partially filled with a transferliquid (9) which is a 120% w/v solution in water of calcium bromide. Ascan be seen, with the U-tube (3) in an upright orientation, the calciumbromide solution (9) fills the bend (11) of the U-tube (3) and extendsinto each leg (13, 15) of the U-tube (3). The calcium bromide solution(9) can be tinted suitably with any water-soluble dyestuff, in thiscase, Eurocert Green S at a level of ca. 0.05% by weight.

Two indexes (17, 19) are inserted within the U-tube (3) and rest at thetwo fronts (21, 23) of the calcium bromide solution (9). The two indexes(17, 19) are made from polypropylene tubing (diameter 2.3 mm., bore 0.4mm. and length 20 mm.). A piece of iron wire (thickness 0.1 mm, andlength 5 mm.) is placed inside each index (17, 19) and the ends sealedby heating and flattening. This produces an index with density near 1gm.cm⁻³.

The indexes (17, 19) have been “balanced” in the expansion liquid (25)i.e the density of the expansion liquid (25) has been adjusted to givebuoyancy to the indexes (17, 19) so that each index (17, 19) can beraised in the limb against a magnetic field provided by magnets (notshown) by movement of the transfer liquid (9). In this example theexpansion liquid (25) is mainly toluene and, in the balancing process1,3-dibromopropane has been added drop by drop to raise the density ofthis liquid suitably.

A small quantity of expansion liquid (9) has been added to leg (15) ofthe U-tube (3) so as to cover index (19) and the stopper (7) fitted. Thereservoir bulb (5) has been fitted, and the whole leg (13) and reservoirbulb (5) filled fully with expansion liquid (25) using a syringe, andthe tap (30) closed.

It will be appreciated that this “test apparatus” is useful forillustrating the viability of all the principles mentioned herein.

The “test apparatus” described above has been tested at temperaturebetween 13.0° C. and 60.0° C. using full immersion in a water bath andwas observed to function appropriately.

In addition to the aforementioned features magnets are used to hold theindexes (17, 19) in position once moved by the transfer liquid (9). Themagnets are positioned behind each leg (13, 15) of the U-tube (3) andtheir distance from the U-tube (3) adjusted in order to allow theindexes (17, 19) to function correctly.

The skilled addressee will understand that in use a temperature scale(not shown) is also provided behind the U-tube (3) so that the maximumand minimum temperatures may be read from the positions of the twoindexes (17, 19) in the U-tube (3).

A more detailed “test apparatus”, similar to a commercialmaximum/minimum thermometer has also been constructed, details of whichare given below:

Normal Thermometric glass is cut into suitable lengths.

An automatic process draws the glass and blows an expansion cavity asshown (FIG. 2). The cavity is left open. A second automatic processheats the opposite end of the stem and by application of filtered airthrough the open ended cavity, the thermometer bulb (40) is formed.

The straight thermometer is now filled with toluene (expansion fluid)(42) in a vacuum filling process.

A reading index (44) (see specification below) is inserted into thethermometer bore and allowed to sink to the bulb (40).

An automatic process, expels the surplus toluene (42) with the bulb (40)above ambient temperature and continues to heat and bend the thermometer(46) into a U-shape.

The bore is then filled with dyed calcium bromide (transfer liquid—seespecification below) (48) care being taken to deposit the calciumbromide on the surface of the toluene.

The U-shaped thermometer (46) is now inverted with bulb (40) being heldabove ambient temperature and surplus calcium bromide (48) is drawn offto a pre-specified distance from the base of the U-bend.

The bore and expansion cavity (49) are then manually filled with toluene(50).

A reading index (51) is inserted into the cavity and allowed to droponto the calcium bromide surface (52).

The thermometer is then cooled below ambient and the expansion cavity(49), (which also contains air due to the contraction of the liquidsused in the bore), is quickly sealed. This allows for compression of theair in the event of the thermometer being heated to a level outside itsmaximum working temperature.

1. Specification for Reading Index

a) Formulation

-   -   Polypropylene PP BFC 1012 Nat:100 gms    -   Magnetite C1312B (Avocado): 36 pph Polypropylene        b) Compounding    -   Materials compounded in torque Rheometer at 180° C. for 8        minutes and granulated using Christy Noris granulator.        c) Strand Extrusion    -   Extrudate production is achieved using a Carter/Beher Acer 2000        capillary Rheometer, fitted with a capillary die of suitable        length and diameter to produce and index of appropriate        dimensions to suit the bore of the thermometer.        d) Extrusion    -   The extrudate is drawn and cut using a linked Gillard        350/30-1800 bench cutter to appropriate lengths.    -   Density of each index approximately 1.15 g/cm³.        2. Specification of Transfer Liquid

Formulation

-   -   120 gm CaBr₂.xH₂O    -   38 ml Supercook Green Colouring    -   62 ml H₂O    -   0.027 ml CF 32 (Rohn & Haas) surfactant    -   Density approximately 1.6 g/cm³.

The U-tube shown in FIG. 2 is then mounted onto a suitable frame, behindwhich, is placed (on each limb) a strip magnet running along the lengthof the limb. These magnets retain the reading index in the position ofmaximum and minimum temperature, to which the transfer liquid meniscihave raised them.

1. A reusable maximum/minimum thermometer comprising an expansion liquid(42) which expands or contracts in response to changes in temperature, atransfer liquid (48) which is moved in response to the expansion orcontraction of the expansion liquid and which is substantiallyimmiscible with the expansion liquid, two indexes (44, 51), and atemperature scale associated with the maximum/minimum thermometer,wherein the indexes are constructed from a plastic material into whichhas been mixed and/or melted a quantity of magnetic powder such as ironfilings, magnetite (Fe₃O₄), and/or strontium ferrite, and wherein theindexes have a density less than that of the transfer liquid, are ableto float on the surface of the transfer liquid, and are moved by thetransfer liquid into resting positions whereby maximum and minimumtemperatures are read off the temperature scale, and wherein thetransfer liquid is a mercury free solution selected from the groupconsisting of inorganic compounds, organometallic compounds, organicliquids, and ionic liquids.
 2. The maximum/minimum thermometer accordingto claim 1 wherein the transfer liquid has a density which is differentfrom that of the expansion liquid.
 3. The maximum/minimum thermometeraccording to claim 2 wherein the transfer liquid has a density greaterthan that of the expansion liquid.
 4. The maximum/minimum thermometeraccording to claim 3 wherein the transfer liquid has a density greaterthan 0.87 g. cm⁻³.
 5. The maximum/minimum thermometer according to claim1 wherein the transfer liquid remains substantially liquid at leastbetween −30° C. and +50° C.
 6. The maximum/minimum thermometer accordingto claim 1 wherein the transfer liquid further displays a low thermalexpansivity; a low toxicity; and/or is coloured or capable of beingcoloured.
 7. The maximum/minimum thermometer according to claim 6wherein the transfer liquid is capable of being coloured by a suitabledye.
 8. The maximum/minimum thermometer according to claim 7 wherein thedye is selected from Aniline Blue, Eurocert Green S or water solublesalts of inorganic complex ions such as the tetra-ammino copper (II) ionor the hexacyanatoferrate ion.
 9. The maximum/minimum thermometeraccording to claim 1 wherein the transfer liquid is a solutioncomprising inorganic or organometallic compounds of elements found ingroups III, IV, V, VI and VII of the periodic table, or mixturesthereof.
 10. The maximum/minimum thermometer according to claim 9wherein the compounds are halogen containing compounds.
 11. Themaximum/minimum thermometer according to claim 1 wherein the transferliquid is a solution comprising ionic compounds.
 12. The maximum/minimumthermometer according to claim 11 wherein the transfer liquid is asolution comprising at least one alkaline earth and/or alkali metalsalt.
 13. The maximum/minimum thermometer according to claim 12 whereinthe solution is an aqueous solution in which said at least one alkalineearth and/or alkali metal salt is dissolved.
 14. The maximum/minimumthermometer according to claim 13 wherein the aqueous solution comprisesan amount of alkaline earth and/or alkali metal salt from between 80%and 400% w/v.
 15. The maximum/minimum thermometer according to claim 14wherein the alkaline earth or alkali metal salt is selected fromhalides, sulphates, hydroxides, carbonates, chlorates, dichromates,chromates, carboxylates, nitrates, nitrites, phosphates, ammoniumcompounds or other salts comprising an alkaline earth or alkali metalion and a cationic species.
 16. The maximum/minimum thermometeraccording to claim 15 wherein the alkali earth or alkali metal salt isselected from CaI₂, CaBr₂, CsBr, CsF, CsOH, Cs₂SO₄, CH₃COOC_(s),KF.2H₂O, HCOOK, KI, KNO₂, RbF, NaClO₄.H₂O, Na ₂Cr₂O₇2H₂O and NaI.2H₂O.17. The maximum/minimum thermometer according to claim 1 wherein theexpansion liquid comprises a hydrocarbon or mixtures of hydrocarbon.